1. Field of the Invention:
This invention relates to a master cylinder which is used with a hydraulic brake system for an automobile. More particularly, it comprises a master cylinder of the portless type having a piston provided with a valve mechanism for establishing or breaking communication between a pressure chamber and an oil reservoir.
2. Description of the Prior Art:
The demand for automobiles of high class has been increasing. As a result, many automobiles now have a brake system provided with an anti-skid device. There is known an anti-skid device of the type which causes a hydraulic fluid to flow back from the wheel cylinders to the pressure chamber in the master cylinder through a pump when the wheels are locked.
There is also known a master cylinder having a primary cup fitted about a piston for opening or closing a compensating port between a pressure chamber and an oil reservoir. A problem is very likely to arise if a brake system including a master cylinder of this type is provided with an anti-skid device of the type as hereinabove described. If the brake pedal is again depressed before it returns to its original position, hydraulic fluid causes the primary cup to flex and flows into the pressure chamber. This fluid produces in the pressure chamber a residual pressure which prevents smooth restoration of the brake pedal to its original position. The repeated use of the brake system gives rise to unreasonably quick wear of the primary cup.
Another type of master cylinder is, therefore, known. Such is of the portless type not having any such compensating port, but which includes a valve mechanism provided on a piston for making or breaking communication between a pressure chamber and an oil reservoir.
A master cylinder of the portless type is shown in Laid-Open British patent application GB No. 2 098 294 A. It includes a piston fitted slidably in a liquid-tight fashion in the bore of a cylinder housing and defining therein a pressure chamber normally in communication with oil pipes leading to wheel cylinders and a fluid supply chamber normally in communication with an oil reservoir. The piston has at one end thereof a recess facing the pressure chamber. A valve seat is provided at the bottom of the recess. A valve member is provided in the recess and is movable axially of the piston away from, or into, contact with the valve seat for establishing or breaking communication between the pressure chamber and the oil reservoir. A first compression spring is provided in the recess for urging the valve member toward the valve seat. A supporting member is connected to the valve member and disposed at the bottom of the pressure chamber. A retainer is fitted on the end of the piston. A connecting rod extends through the supporting member and the retainer and is movable by a predetermined distance axially of the piston. A second compression spring is disposed between the supporting member and the retainer for urging them away from each other to cause the connecting rod to engage them to move the valve member away from the valve seat by a predetermined distance.
Another master cylinder of the portless type is disclosed in Japanese Laid-Open patent specification No. 102762/1980. It includes a piston which is substantially identical in construction to the piston disclosed in the British application hereinabove referred to. A valve rod is connected to the piston and is movable axially of the piston. The valve rod is provided with a valve member which is movable away from, or into, contact with the valve seat on the piston for making or breaking communication between the pressure chamber and the oil reservoir. A compression spring is provided for urging the valve member toward the valve seat. The piston has an axially elongated hole. A stopper pin extends through the hole and is fixed to the cylinder housing. When the piston is in its inoperative position, the pin engages the end of the leg of the valve rod to move the valve member away from the valve seat by overcoming the force of the spring.
Both of these master cylinders are, however, still unsatisfactory. It is often the case that the residual pressure produced in the pressure chamber as hereinbefore stated may cause the valve member to be stretched elastically and stay at rest on the valve seat, even when the piston is brough back to its inoperative position. If the valve remains closed, the residual fluid cannot be released from the pressure chamber, but disables the brake system so as to be deactuated completely. This problem may be solved if the valve member and the valve seat have a greater distance therebetween when the piston is in its inoperative position. This solution, however, results in an increased idle stroke of the piston and therefore of the brake pedal.
The master cylinder disclosed in the Japanese application has still another disadvantage. The presence of the stopper pin complicates the work of assembling and disassembling the master cylinder. The circumferential position of the piston is critical to ensure the alignment of its elongated hole with the pin. The pin must be removed when the piston or other parts of the master cylinder are changed.